1. Field of the Invention
The present invention relates to a method for manufacturing an ultrafine carbon fiber such as a graphite nanofiber, a carbon nanofiber, a carbon nanotube, tubular graphite, a carbon nanocone having a thin and sharp edge, corn shape graphite, or the like and a field emission element having the ultrafine carbon fiber.
2. Description of the Related Art
In recent years, an ultrafine carbon fiber is expected to be applied to various devices. Accordingly, various methods for manufacturing an ultrafine carbon fiber are examined.
An arc discharge method, laser vaporization, CVD (Chemical vapor deposition), or the like is used for a typical manufacturing method of an ultrafine carbon fiber (Reference 1).
There are an ultrafine carbon fiber such as a carbon nanotube (hereinafter, referred to as a CNT), a carbon nanofiber, a graphite nanofiber, tubular graphite, a carbon nanocone having a thin and sharp edge, or corn shape graphite, a fullerene, and the like as a typical example of an ultrafine carbon fiber.
A CNT refers to cylindrical graphite having a nanometer size. There are a single-walled nanotube and a multi-walled nanotube as a CNT. A single-walled nanotube is a tube in which a sheet of a graphene sheet (a carbon hexagonal net plane of a single atomic layer) is cylindrically closed and the diameter is about from 1 nm to 10 nm and the length is from 1 μm to 100 μm. A multi-walled nanotube is a tube in which cylindrical graphene sheets are laminated, and the outside diameter is from 5 nm to 50 nm, the diameter of a central cavity is from 3 nm to 10 nm, and the length is from 1 μm to 100 μm.
A CNT has a sharp edge and a needle shape, is thermally and chemically stable, mechanically strong, and has properties of having conductivity or the like. The CNT is applied to a probe of a Scanning Probe Microscope (SPM), a field emission element of a field emission display device (hereinafter, referred to as a FED), and a channel region of a FET (Field Effect Transistor). In addition, a negative electrode material of a lithium battery, a gas occluding substance, or the like is studied by making a use of a structure in one-dimensional pores having substantially large space in a tube or between tubes.
An ultrafine carbon fiber is applied as a field emission element of a field emission display device since it has a low work function and has negative electron affinity.
As described in Reference 2, a method for forming a metal dot over a silicon substrate surface that is partly exposed, applying a magnetic field to the metal dot with an electromagnet in a vertical direction to the silicon substrate surface, and forming an electron emission portion by growing a CNT between the metal dot and the silicon substrate while absorbing the metal dot is used as an example of a manufacturing method using an ultrafine carbon fiber for a field emission element of a FED (Reference 2).    Reference 1: “Carbon nanotube: an expected material development,” CMC Corp., Nov. 11, 2001, pp. 3–4.    Reference 2: Japanese Patent Laid-Open No. 2000-86216
However, it is difficult to control separately a diameter and a length of an ultrafine carbon fiber in the conventional manufacturing method, and has a problem of large variation in a diameter. Generally, when an ultrafine carbon fiber such as a CNT is formed using a metal film as a catalyst, it is said that the diameters depend on the diameter of the metal film. In Reference 2, a CNT is formed using a metal dot as a catalyst; however, a step of forming a metal film of from several nm to several ten nm is complicated, that is, it is difficult to control a diameter of a CNT, which is problematic.